Valve Gear
Shape of the cam determines: 3 things
1. Point of opening and closing 2.Velocity of cam opening 3.Amount of valve lift
What do Valve Guide Inserts do?
1. Provides for easy replacement when worn 2.Allows a more suitable material with wear resistant qualities to be used
Advantages of chain drive camshafts are:
1. reliable- chains can take more abuse and not affected as much by foreign particles. 2.Accuracy-chains can be tightened to compensate for wear 3.Use of the chain drive permits camshaft to be positioned higher on the engine-reduces timing errors due to elasticity
When two or more valves on a cylinder must be opened/closed simultaneously, one rocker arm may be used to operate two valves. This is accomplished by using:
1.A forked rocker arm (TVSOM 6M601) 2.A valve Bridge
Methods of driving the camshaft include:
1.Belt drives 2.Gear drives
Valve gear includes:
1.Camshafts 2.Cams A. Intake B. Exhaust C. Fuel injection 3.Cam followers 4.Push Rods 5.Valve bridges 6.Valves A. Intake B. Exhaust
Push Rods
1.Connects the cam follower to the rocker arm 2.Weak link in valve gear
Shape of cam flanks determines velocity of opening or closing: 2 Things
1.If convex or concave, slower opening and closing 2.If straight, faster opening and closing
Valve requirements are ? (6)
1.Must be opened as quickly and widely as possible 2.Must remain open as long as possible 3.Must close as soon as possible 4.Must be made as light as possible 5.Must be made as robust as possible 6.Must be able to withstand high temperatures
If valve lash is excessive:
1.Noisy operation 2.Loss of power - due to incorrect valve timing because valve opens later and closes earlier 3.Excessive wear
Hydraulic lifters allow for zero valve lash at all load conditions so it
1.Reduces pounding of valve stem 2.Reduces pounding of valve seat 3.Reduces valve failure by approximately 50% 4.Not used in most large engines because of added cost and problems with lube oil contamination
Cam follower types
1.Roller-most common 2.Flat-Used in smaller and high-speed engines
The number of cams per cylinder is determined by: 2 things
1.The engine operating cycle 2.The method of scavenging in a 2-stroke cycle
A valve cage consists of hollow cylindrical castings containing:
1.Valve 2.Valve guide insert 3.Valve seat insert 4.Valve springs, keepers, and retainers 5.Cooling passages
If valve lash is insufficient:
1.Valve will not seat properly 2.Valve leakage and burning of the valve/valve seat will occur 3.Loss of compression and thus loss of power
Valve rotation is typically applied to exhaust valves and
1.ensures that the valve is kept at an even temperature so that it does not distort and leak by 2.Valve rotation also helps dislodge any build-up of deposits on the valve and seat
Methods of Camshaft Construction Include: (2)
1.integrally forged 2.separately fabricated with cams bolted, keyed, or hydraulically shrunk to camshaft
In rocker arms, One end in contact with valve stem, the other in contact with either
1.push rod 2.Cam in Overhead Cam design
To prevent spring surge,
2 to 3 smaller springs may also be used since small springs have a high natural frequency of vibration
What are The most common angles?
30 degrees and 45 degrees
What uses Belt Drives?
Automobiles
Larger camshafts may be What to reduce weight?
Bored Hollow
Cams are case-hardened to increase what?
Durability
Which Valve experiences the most severe operating conditions?
Exhaust Valve
What is the 45 degree angle good for?
Exhaust Valves because it prevents valve seat deposits but may increase wear due to valve pounding
Pumping Loss:2 stroke vs 4 stroke
Exhaust valves are important on 2 and intake important on 4
Valve Spring Seat:
Located at the lower end of the spring on the cylinder head
Valve Seat Width
Seat width is critical to valve life - it is determined by the manufacturer and increases in proportion to engine size
resonance will cause
Spring Surge
What uses Gear Drive?
Sulzar Diesel engine
Valve lash is defined as the clearance between:
The valve stem and the rocker arm or the push rod and the rocker arm
What are Valve Cages used for?
To allow easier removal for replacement or reconditioning in large engines
Rotocap is used in which type of engine?
Trunk type
What is stellite?
a cobalt-based alloy that resists the tendency most metals have to soften as temperatures increase
Spring surge is
a situation where the center coils of the spring oscillate due to their own inertia
Cam
an eccentric projection on a revolving disk used for opening and closing a valve through intermediate valve gear
What does a 2 stroke cycle cycle have?
at least one cam per cylinder for fuel injection and may have one or more cams for exhaust, depending the number of exhaust valves
The purpose of the valve spring is to
close the valve
Intake valves always operate in relatively cool temperatures and can therefore be constructed of
common ferritic steel
Valve cages also
eliminates the need to disturb the cylinder head and liner simply to change a valve
Arrangement of cams on camshaft determines?
engine firing order
Rocker Arms
held on its pivot by a shaft or guide which allows it to pivot when actuated by push rod or cam
What is Camshafts Typically fabricated of?
high strength alloy steel forging
What is the 30 degree good for?
intake valves
Camshaft speed in a 4-stroke cycle engine
is ½ the speed of the crankshaft
Camshaft speed in a 2-stroke cycle engine
matches the speed of the crankshaft
Rotocap is a
mechanical device which produces valve rotation by a small amount as the valve opens
A 4-stoke cycle engine typically has
one fuel injection cam, one intake cam, and one exhaust cam
Valve springs have a closing force directly
proportional to the amount they are compressed
valve is rotated as it reseats, a light grinding action takes place. This
removes deposits and ensures a good seal
if the amplitude of vibration increases with each application of force to its natural frequency it causes
resonance
Exhaust valves are typically made of
silicon chromium (Silchrome) steel or some other form of austenitic steel such as Nimonic
The spring retainer is combined with a
split keeper or "collet" to grasp the valve stem
To increase the wear resistance properties of exhaust valves,
stellite may be used to build up the face of the valve or the valve seat
What is the Valve Seat Angle?
the angle between the valve seat surface and the cylinder head surface
What is Interference Angle?
the angle of the valve face is made ½ to 1 degree less than the angle of the seat
What drives the camshaft?
the crankshaft
A disadvantage of the interference angle is that
the narrow contact area results in less heat transfer from the valve face to the valve seat
natural frequency of vibration depends on
the shape and mass of the spring
Valve Spring Retainer:
the upper spring seat and consists of a hardened washer shaped to fit the spring